926 RESPIRATION 



Most of the enzymes of the cycle are demonstrable in extracts of 

 Zygorhynchus moelleri (201), and isotope distribution experiments 

 indicate that the cycle plays a major role in energy metabolism (202). 



Apart from these more intensively studied species, individual en- 

 zyme activities have been determined in many other fungi and 

 actinomycetes. These include oxalacetic decarboxylase (87, 306), a - 

 ketoglutaric acid oxidase (46), "oxalosuccinic decarboxylase" (183), 

 enzyme systems oxidizing succinic acid (46, 89, 137, 182, 216), and 

 isocitric dehydrogenase and condensing enzyme (12). 



In conclusion, it is evident that the enzymes of the citric acid cycle 

 are of quite general occurrence among fungi and actinomycetes and, 

 indeed, that further enzyme surveys can add little to our understand- 

 ing. The more important question is the physiological role of the 

 citric acid cycle. Two such roles may be envisaged: provision of 

 energy to the cell, and provision of carbon skeletons for amino acid 

 synthesis. The second of these is reasonably certainly a real function 

 of the cycle in Neurospora crassa, Penicillium chrysogenum , and 

 Streptomyces griseus. A role in energy metabolism is more difficult to 

 establish, but would be of great interest. Citric acid accumulation 

 seems to result directly from an interruption of the cycle, under special 

 nutritional conditions, at the citric acid stage. 



We must also recall from Chapter 6 that isocitritase, catalyzing the 

 conversion of isocitrate to succinic and glyoxylic acids, is known to 

 occur in fungi; its activity may contribute to some of the complexities 

 of the metabolic data. 



10. DICARBOXYLIC ACIDS IN RESPIRATION 



Apart from their role in the citric acid cycle, it has long been 

 speculated that the 4-carbon dicarboxylic acids (succinic, fumaric, 

 malic, and oxalacetic) may be components of another respiratory cycle 

 in which acetate is oxidized to carbon dioxide. The initial reaction 

 of such a cycle would be the reductive condensation of acetate, first 

 suggested by Thunberg (298): 



2 CH3COOH -» HOOC— CHo— CH 2 — COOH + 2 [H] (12) 



Further reactions can be imagined whereby the succinate so formed is 

 converted to 1 mole of acetate and 2 moles of carbon dioxide, the 

 acetate re-entering the cycle. The net effect would be the oxidation 

 of one mole of acetate for each turn of the cycle. Possible dicarboxylic 

 acid cycles are described by Ajl (1), Ochoa (222), and Umbreit (307); 

 Siegel (262) discusses energy relations. 



